The present invention relates to the art of electronic packaging and more specifically to components useful in making electrical connections to microelectronic elements such as semiconductor chips, and to method of manufacturing such components.
Certain techniques for making semiconductor chip assemblies and similar microelectronic assemblies employ releasably attached leads. One such process is disclosed in commonly assigned, U.S. Pat. No. 5,518,964, the disclosure of which is hereby incorporated by reference herein. In certain preferred embodiments described in the '964 patent, a first element such as a dielectric layer in a connection component is provided with a plurality of elongated, flexible leads extending along a surface of the first element. Each lead has a terminal end permanently attached to the first element and has a tip end offset from the terminal end. The tip ends of the leads may be releasably secured to the first element. A second element such as a semiconductor chip having contacts thereon is engaged with the first element or connection component, and the tip ends of the leads are bonded to contacts on the chip or second element. The elements are then moved away from one another so as to deform the leads and provide vertically extensive leads extending between the first and second elements, i.e., between the chip and the connection component. A compliant material may be introduced between the chip and the connection component.
The resulting structure allows the chip to move relative to the connection component without substantial stresses on the leads, and thus provides compensation for thermal expansion and contraction. The preferred structures can be readily tested and can be mounted on a support substrate such as a printed circuit panel or the like. In the preferred processes, many leads can be deformed simultaneously.
In certain embodiments disclosed in the '964 patent, the tip end of each lead is bonded to the surface of the first element by a small spot of a base metal such as copper interposed between the tip end and the surface. Typically, such a spot is formed by a process in which the leads are formed from an etch-resistant metal such as gold overlying a continuous layer of the base metal. The leads have wide portions at the tip and terminal ends. The component is then subjected to an etching process so as to undercut the lead and remove the base metal from beneath the etch-resistant metal at all locations except at the terminal end and at the tip end. At the tip end, the most, but not all of the base metal is removed from beneath the etch-resistant metal, leaving a very small spot of the base metal. The strength of the bond between the tip end and the connection component surface is effectively controlled by the size of the spot. Thus, although the base metal may provide a relatively high bond strength per unit area or per unit length, it may still provide a weak attachment at the tip end of the lead end of the first element surface.
As described in certain embodiments of U.S. Pat. No. 5,536,902, the disclosure of which is hereby also incorporated herein by reference, a connection component may incorporate a support structure such as a polyamide or other dielectric layer with one or more gaps extending through such layer. Preferably, the support structure incorporates one or more flexible or compliant layers. The connection component may further include leads extending across the gap. Each lead has a first or terminal end permanently secured to the support structure on one side of the gap, and a second end releasably attached to the support structure on the opposite side of the gap. In preferred processes as taught by the '902 patent, the connection component is positioned on a semiconductor chip or other microelectronic element. Each lead is engaged by a bonding tool and forced downwardly into the gap, thereby detaching the releasably connected second end from the support structure. The leads are flexed downwardly into the gap and bonded to the contacts on the chip or the microelectronic element. Preferred connection components and processes according to the '902 patent also provide highly efficient bonding processes and very compact assemblies. The finished products provide numerous advantages such as compensation for thermal expansion and contraction, ease of testing, and a compact configuration.
Other structures disclosed in the '902 patent and in the '964 patent employ a frangible lead section connecting the releasable end of each lead to another structure permanently mounted to the support structure or first element. Frangible sections can also provide useful results. However, such frangible elements are most commonly constructed by forming narrow sections which are themselves formed using the same type of the photo-etching or selective deposition processes as are used to form the lead itself. The minimum width of the narrow section can be no less than the smallest width formable in the process. As the other portions of the lead adjacent to the narrow section must be wider than the narrow section, these other portions must be larger than the minimum attainable in the process. Stated another way, the leads made by such a process generally are wider than the minimum line width attainable in the formation process. This limits the number of leads which can be accommodated in a given area.
In other embodiments disclosed in the '902 patent, the first or permanently mounted terminal end of a lead may have a relatively large area, whereas the second or releasably mounted end of the lead overlying the support structure may have a relatively small area, so that such second end will break away from the support structure before the first end when the lead is forced downwardly by the bonding tool. This arrangement requires careful control of the dimensions of the ends to control the area of the bond between the lead end and the support structure and also requires a lead wider than the smallest element formable in the process.
As described in U.S. patent application Ser. No. 08/547,170, now U.S. Pat. No. 5,763,941, the disclosures of which are also hereby incorporated herein by reference, a connection component may incorporate a dielectric support structure and a plurality of leads extending along a surface of the dielectric support layer. Each lead has a first region that is permanently connected to the dielectric layer, and a second region which is releasably connected to the dielectric layer such that the second region can be detached from the dielectric layer by moving the lead away from the dielectric layer. The first and second regions are preferably disposed at opposite ends of the lead. In preferred embodiments, the second region is attached to the dielectric layer through a release interface having a peel strength of less than about 0.35×106 dynes/cm. In one embodiment in the '170 application, the release interface is formed by depositing a release metal onto the polymeric materials of the dielectric layer and abutting the metallic surface of the lead thereto. According to another aspect of the invention of the '170 application, the release interface can be formed in a deposition process in which the steps normally taken to assure good adhesion are omitted. Such steps include oxidation of, and high energy sputtering of nickel onto, the polymer layer. In yet another embodiment, the release interface incorporates a first release metal securely bonded to the dielectric layer and a second release metal on the lead, the first and second release metal being weakly bonded to one another.
In certain embodiments of commonly assigned U.S. patent application Ser. No. 08/634,784, which is incorporated herein by reference, Smith et al. disclose a connection component for a semiconductor assembly comprising a dielectric support layer having a gaps therein. The component also includes a plurality of strip-like leads that extend across the gaps and are bonded to a conductive material on the dielectric layer. The strip-like leads may be formed from lengths of wire and may be bonded to the conductive material using a bonding process such as thermosonic bonding.
Although these packages offer substantial improvements over the prior art in the packaging electronic devices, the force needed to peel the leads from the support structure may be excessive, especially for high lead count devices and for wafer level packaging.
Accordingly, further improvements in releasable lead structures and method of making the same would be desirable.